This invention relates to a novel liquid crystal composition. More particularly, it relates to nematic liquid crystal mixtures which exhibit glassy structures at low temperatures.
Various liquid crystal compositions have been disclosed in the prior art. A typical desired specification for a field effect nematic liquid crystal display composition includes a temperature range of from about 0.degree. to about 60.degree. C, with the lower temperature indicating the melting point of the material and the upper temperature indicating the clearing or nematic isotropic temperature. Second, a threshold voltage of from about 1.0 to about 1.5 volts and an operating voltage of from about 1.5 volts to about 2.5 volts is desired. Third, the composition should exhibit sufficient optical contrast for display purposes when incorporated in a liquid crystal display structure. Fourth, the lifetime of the liquid crystal composition should be at least about 5 years. Fifth, the resistivity of the composition should be at least about 5.times.10.sup.9 ohm-centimeters.
Schiff bases have been widely used as liquid crystal compositions. Typical of the Schiff bases employed for this application are those disclosed in U.S. Pat. No. 3,792,915. However, the Schiff bases tend to be unstable and will hydrolize rapidly unless careful precautions are taken to seal the package containing them against water and ultraviolet light.
Phenylbenzoate ester liquid crystal compounds are also known in the art as described in U.S. Pat. No. 3,876,286. While such compounds are known to be highly stable, they generally have higher melting points and narrower nematic temperature ranges than Schiff bases.
Some work has also been done with mixtures of Schiff bases and phenylbenzoate ester liquid crystal compounds, as disclosed in U.S. Pat. No. 3,881,806. However, many such mixtures lack sufficient chemical stability for use in liquid crystal displays. Furthermore, as disclosed in the above mentioned U.S. Pat. No. 3,881,806, such mixtures have not hitherto produced improvement in low temperature operating characteristics.
In many potential applications for liquid crystal displays, the display may be exposed to temperatures below their normal operating temperatures. For example, if a liquid crystal display is used in a wristwatch or in the instrument panel of an automobile, it may be exposed to below zero temperatures from time to time, especially in northern climates. If a liquid crystal composition freezes, this results in loss of contrast in the display in later operation, due to disruption of surface orientation in the display.